The invention concerns a mold for vulcanization of tire casings by direct action of a fluid on the inner walls of a tire casing having two beads.
In the generally used methods of vulcanization of rough tire casings, the blank is shaped and laid on a vulcanization mold by means of an inflatable and heatable rubber bladder, which can be thick and is commonly called a vulcanization bag, or which can be thin and is then called a vulcanization membrane.
The problems associated with those methods are due, on the one hand, to the device or press necessary for the operations of placement and removal of the bladder and, on the other, to the nature of the inflation bladder itself. In fact, the bladder is sensitive to wear due to multiple use and has to be replaced after a number of curings have been performed, a replacement operation that is, on the one hand, expensive and, on the other, causes a downtime of the vulcanization press that is reflected in the cost of the tire, especially considering that the rubber bladder is a heat screen and does not allow for vulcanization as rapidly as might be desired. Furthermore, possible heterogeneities of the bladder are encountered, on completion of vulcanization, in the finished tire and directly affect production. In the shaping process, for example, the thinner portions of the inflatable bladder stretch more than other regions and produce defects on the tire casing which are manifested by excessive expansion of the casing mounted on a rim at the corresponding places on said rim. It can be further mentioned that the inflatable bladder contains on its outer surface a series of grooves intended for evacuation of the air trapped between the tire casing and the bladder. In the vulcanization process, said grooves can cause deformations of the interior layers of the carcass reinforcement. Furthermore, and in particular, in the case of use of a membrane, numerous wrinkles exist on said membrane and the lack of uniformity thus created is reflected on the tire itself.
To overcome the abovementioned disadvantages, it was decided, to abandon the use of an inflation and vulcanization bladder and to cause the vulcanization fluid to act directly on the inner wall of the tire casing. However, in order to meet the requirements of correct positioning of the beads and tightness of the mold, the beads of the casing must be pressed in the axial direction and by means of circumferential press plates against the corresponding surfaces of the vulcanization mold, as described, for example, in French Patent 1,345,102.
In those known vulcanization systems without the inflation bladder, the use of press plates or rings creates, first of all, mechanical difficulties, the placement of such plates after closing of the press demanding a high degree of mechanical precision and, secondly, heat problems, the part of the bead axially compressed escaping the action of the heating fluid, which obviously produces a vulcanization temperature variation in the bead itself and therefore creates a notable vulcanization difference, with a heterogeneous quality of batches and, consequently, a markedly diminished endurance of the beads.
The invention is intended to overcome all these disadvantages.
According to the invention, the vulcanization mold for a tire casing having two beads for vulcanization by direct action of a fluid on the inner walls of said casing, said mold comprising two crowns for molding the outer surface of each of the beads, is characterized in that each molding crown successively comprises, axially from the center of the mold to the outside of the latter, a first annular section having an outer bearing surface to support the corresponding bead, on shaping of the rough tire casing, and a second annular section, axially adjacent said bearing surface and axially more outward from the latter, providing for the molding of said bead and the tightness of the mold.
The presence of those two separate annular sections, one responsible for the shaping operation and the other for the molding operation, makes it possible to avoid resorting to the use of press plates or rings in contact with the interior surface of the beads, nevertheless ensuring a good positioning of the beads and, therefore, of the tire casing in the mold and guaranteeing the tightness of the latter.
The second section successively comprises, axially from the center of the mold to the outside of the latter, a circumferential bearing in contact with the first section, a circumferential groove and a roughly truncated cone-shaped portion, the bearing having a roughly cylindrical surface, whose diameter is greater than the minimum interior diameters of the rough and cured tire casing. The minimum diameter defined by the bead of said casing is designated by the interior minimum diameter of the tire casing.
The presence of this bearing makes it possible, on the one hand, to ensure, on shaping of the tire casing, that the beads are borne on the first sections of the molding crowns and, on the other, to ensure, in cooperation with the groove, the maintenance of tightness of the mold. Preferably, no vent is provided, in contrast to what is often done, on the entire radially outer surface of said crowns. The surface of the crowns of the mold is totally airtight.
Other characteristics and advantages of the invention will be apparent from the description which follows of a working example of a vulcanization mold according to the invention, with reference to the attached drawings: